In recent years, for the purpose of fuel efficiency improvement and conformity to environmental standards, development has been made on a vehicle that has a generator-motor mounted thereon to perform a so-called idle stop for stopping an engine at the time of stopping the vehicle and restarting the engine at the time of moving the vehicle. Such vehicle generator-motor is required to be compact, low in cost, and high in torque, and hence a field winding generator-motor is often used.
In general, in the field winding generator-motor, the inductance of a field winding is larger than the inductance of an armature winding. Thus, if energization to the armature winding and energization to the field winding are simultaneously stopped in order to stop the driving, because the reduction rate of the field current is slower than that of the armature current, an induced voltage may be generated by a residual field magnetic flux to cause an unintended power generating state. In such case, excess generated electric power may adversely affect a battery or other devices, or excess braking torque may be output to adversely affect engine control as well.
To deal with this, in Patent Literature 1, the following method is proposed. After an instruction to stop driving is given, processing of reducing a field current is performed while an armature is continued to be energized. Then, after the field current decreases to a certain level, the energization to the armature is stopped to prevent power generation.
In Patent Literature 1, this field current level is determined based on an rpm of the generator-motor and a voltage between input terminals (hereinafter referred to as “B-terminal voltage”) or a battery voltage so as to have a value in a range where an induced voltage may fall below the B-terminal voltage or the battery voltage.